1. Field of the Invention
The present invention relates generally to semiconductor processing equipment and more particularly to plasma sources that can be operated at high powers for increased processing rates.
2. Description of the Prior Art
Microwave energy is conventionally coupled to gas flows to generate plasmas useful in semiconductor wafer processing. Prior art equipment couple as much as 1200 watts of microwave energy centered at 2.45 GHz from a ringed "applicator" through and into a quartz plasma tube that passes through the rings.
Carl F. Weissfloch, et al., describe one such applicator in U.S. Pat. No. 3,814,983, issued Jun. 4, 1974, and state that the use of microwave plasmas has been severely limited by the small size of plasma volumes achievable with conventional microwave applicators, namely antennas, waveguides, and cavity resonators. So a slow wave structure illustrated in FIGS. 6-8 of such patent is offered. A thirty-six inch long semi-radiant slow wave applicator is described that operates in the degenerate half wavelength (.lambda./2) mode. Slow wave structures such as microwave applicator are supposedly dividable into two types, resonant slow wave structures and traveling slow wave structures. A rectangular waveguide 2 transitions to the slow waveguide structure with a doubly tapered inner conductor 24 that connects to a tapered parallel plane transmission line 25. A set of twelve parallel bars 26, 31 are arranged in a fence-line in FIG. 6 and in a circle in FIGS. 7 and 8. Each bar 26, 31 terminates in a top shorting plane 29, 34 and at the opposite ends in a bottom shorting plane 29, 34. Some prior art equipment uses fourteen such bars in a circle. A central hole in each of the ring shorting planes 34 allows for the insertion of a plasma tube in which the microwave energy is delivered. In between the planes 29, 34, the odd numbered bars 26, 31 connect together with a strap 27, 32 and the even numbered bars 26, 31 connect together with a strap 28, 33. The transmission line 25 is connected to each of the straps 27, 32 and 28, 33 and enters perpendicular to the bars 26, 31.
In practice, such prior art structures have not worked very well. The power delivery is usually very uneven and what energy is delivered concentrates near the first three sets of bars 31 closest to the feed in circular structures. The power delivered by the microwave system is usually limited to 1200 watts because spot heating of the quartz, sapphire or ceramic plasma tubes is too severe to be handled adequately by a cooling system. The terminating impedance presented to the waveguide and microwave source has not been good, and much of the power input is reflected back. The relatively close physical spacing of adjacent bars 26 and 31 on opposite phases carried by the straps 27, 28, 31 and 32 tend to arc together and pitting results.